willow90

A customer of ours manufactures air filters for all types of engines on all types of machinery, from JCB's and generators to performance cars. Their general rule of thumb is that a manufacturers' spec filter is usually correct. Free-flow filters have bigger 'holes' in them to allow better breathing. This allows more rubbish through which will result in greater engine wear etc. Their advice to me for my 300 90 was 'use a standard filter but change it every three months or so, more often than that if used in very dirty / wet conditions'. You can get standard filters cheaply enough on-line, so my advice would be to buy half a dozen at once to save on delivery costs. Store them somewhere dry, but if you think they might get wet or dirty in storage, seal 'em up in a plastic bag.